In the following, the configuration of a general field emission type display device will be explained with reference to FIG. 1.
The field emission electrode (cathode) is usually formed on the back glass substrate, with there being a space set between the front and back glass substrates. The space is in an ultrahigh vacuum state so that electrons can easily pass from the cathode (emitter) electrode, with a light-emitting layer coated with a phosphor, to an anode electrode formed on the opposite (front substrate) side. The two glass substrates are sealed with a glass frit. Band (belt)-shaped (in another words, it is long narrow strip) gate electrodes (used for extracting electrons from the emitter, releasing electrons to the anode electrode, and controlling the quantity of the electrons released from the emitter by controlling the voltage applied to the gate electrodes) are formed on a dielectric layer in such a way that the gate lines are in parallel with each other and perpendicular to the cathode electrodes. Gate openings with a prescribed diameter that constitute electron releasing parts are formed at the crossing parts of the gate lines and the cathode lines. Emitters made of, e.g., carbon nanotubes are formed at the gate openings. By applying a high voltage between the transparent anode electrode and the cathode electrode on the display glass substrate, electrons pass into the vacuum from the emitter electrode and collide with the phosphor surface. Energy levels are changed by the level of electrons passing to the phosphor surface, this results in a display of the light emission phenomenon.
Conventionally, a gate electrode is formed using lithographic technology to etch a thin film formed by deposition/sputtering/chemical vapor deposition (CVD) Mo, Cr, etc. (see Japanese Kokai Patent Application No. Hei 9[1997]-35670) or formed using the photolithographic method or wet etching method (see Japanese Kokai Patent Application No. Hei 10[1998]-21458). The electrodes of a field emission type display device, plasma display, or other flat display devices can also be formed as follows. After a photosensitive silver printing paste is used to form a film on a glass substrate, UV exposure and development are performed to pattern the film into the electrode shape, followed by sintering (see Japanese Kokai Patent Application Nos. 10[1998]-144208 and Hei 10[1998]-144210).
However, gate electrode formation using the aforementioned thin film technology is only used for the fundamental research and development of the field emission type display device, and the best film thickness that can be realized in practical applications is merely 1 μm. Although there are various opinions, the resistivity of the gate electrode is usually desired to be 100 μohm·cm or lower. In the case of commercializing a large-scale field emission type display device, use of the thin film technology might significantly increase the equipment investment and the manufacturing cost. If the cathode electrode, insulating layer, gate electrode, and other parts formed on the back substrate can be formed with thick film technology, the manufacturing cost can be cut significantly, and, if necessary, parts with a thickness of several μm can be formed. Based on this point of view, the potential application of the thick film technology as much as possible in the same way to a PDP (plasma display panel) is firmly rooted.
In the following, several restrictions concerned with the gate electrode of a field emission type display device based on thick film technology will be explained. First of all, the sintering temperature is low. For PDP, a temperature in the range of 500-600° C. can be used for sintering the thick film parts. The field emission type display device, however, has a higher pattern precision requirement than PDP. Therefore, the thick film parts should be sintered at a temperature below 550° C., preferably around 500° C. Second of all, the thickness of the insulating layer in contact with the gate electrode is preferably 10 μm or less, and there is a short circuit or voltage breakdown occurring between the upper gate electrode and the lower cathode electrode. The conventional various types of thick film insulators have a thickness in the range of 20-50 μm, and a thick-film silver electrode can be used. Even with PDP, photosensitive silver electrode material is very popular, and photosensitive thick-film silver electrode material is used as the bus electrode below the transparent dielectric on the front panel. However, it is questionable whether silver can be used for the upper gate electrode with the thickness of the insulating layer being only half (10 μm) of that in the conventional technology, even if the lower cathode electrode is a thin film. The reason is that silver will be diffused into the thin insulating film that constitutes the base of the gate electrode. As a result, the breakdown voltage characteristic and the insulation property of the insulating layer will be deteriorated.
In addition to silver, other conductive components include ITO, nickel, copper, gold, aluminum, tungsten, carbon, etc. First of all, gold, tungsten, and nickel are inappropriate due to the fact that sintering should be carried out at around 500° C. Aluminum is inappropriate because of its powder cost and handleability. ITO is inappropriate due to its electroconductivity. Carbon is inappropriate in consideration of the stability in vacuum. Thus, copper is the only choice left that can be used for the gate electrode.
If copper is sintered in an oxidizing atmosphere, an oxide film will be formed on the surface, inhibiting the sintering and preventing the action of copper as a conductor. Therefore, copper must be sintered in a reductive atmosphere. As described in International Laid-open Patent WO 01/99146 proposed by the present applicant, in the manufacturing method of a field emission type display device, a thick film composition containing carbon nanotubes is used for the emitter electrode of the cathode in a step after forming the gate electrode, and it is necessary to sinter the aforementioned composition in a reductive atmosphere. Consequently, investment in equipment for a reductive-atmosphere sintering furnace that is particularly used for forming the gate electrode becomes unnecessary. This is not considered to be a significant restriction.
Also, in consideration of the sintering of copper, as described in Japanese Kokai Patent Application No. Hei 10[1998]-294018, sintering can be performed at 500° C. or lower by carrying out calcination under vacuum, followed by secondary calcination in an oxidizing atmosphere and the main sintering in a reductive atmosphere. Currently, however, it is still technically unrealistic and very difficult to sinter a large glass substrate.
Additionally, use of the compositions of this invention is also conducive to fabricating a lighting device. Such a device comprises (a) a cathode including an electrode as outlined in this work and (b) an optically transparent electrically conductive film serving as an anode and spaced apart from the cathode, and (c) a phosphor layer capable of emitting light.
The patterned and/or layered electrodes provided by this invention can be used in the cathodes of electronic devices such as triodes and in particular in field emission display devices. Such a display device comprises (a) a cathode using an electron field emitter, (b) an optically transparent electrically conductive film serving as an anode and spaced apart from the cathode, (c) a phosphor layer capable of emitting light upon bombardment by electrons emitted by the electron field emitter and positioned on or adjacent to the anode, and between the anode and the cathode, and (d) one or more gate electrodes disposed between the phosphor layer and the cathode. The use of the compositions of this invention to fabricate the cathode, including the insulator, and gate structures is readily adapted to cathodes of large size display panels.